Plastic optical fibers are being used mostly in areas such as short-distance optical signal transmission and sensors, because they are more flexible than silica optical fibers, and besides their end-face processing or connection is easy if they are fabricated to have a large diameter and high numerical aperture.
Of plastic optical fibers having been put to practical use, plastic optical fiber strands are being widely used which use as a core material a highly clear resin, such as a copolymer composed mainly of methyl methacrylate (poly(methyl methacrylate)-based resin) or polycarbonate resin, and as a cladding material a vinylidene fluoride-based copolymer or methacrylate fluoride-based copolymer.
These plastic optical fiber strands are generally used in the form of plastic optical fiber cables, which are produced by applying to the outside of the cladding layer a coating layer of, for example, a low-density polyethylene, poly(vinyl chloride) or polyamide so as to prevent the degradation of their optical properties due to scratches or the like.
In recent years, attempts have been made to use plastic optical fibers in a portable electronic equipment that includes two casings both having a hinge structure section to connect signals between the two casings (see Patent Document 1). The existing plastic optical fiber cables using plastic optical fiber strands whose outside diameter is 0.7 mm, however, have a performance limitation such that the cables break, when subjected to bend test with a bend radius of 2 mm and at a bending angle of ±90°, after 100,000-time or less repeated bending.
One possible approach to improving the flexure resistance in the bend test is to decrease the outside diameter of the plastic optical fiber strands and bundle the plurality of plastic optical fiber strands to compensate for the light attenuation. Plastic optical fiber cables have been proposed based on the above idea and are on the market which are produced by bundling 4 single-core plastic optical fiber strands as thin as 0.265 mm in outside diameter and coating the bundle with polyethylene. However, cables produced by using thin plastic optical fiber strands in a bundle also create problems of: their being difficult to connect, because their coating layer must be removed when they are connected and the thin plastic optical fiber strands become apart; and causing a large optical loss due to connecting.
There have also been proposed multi-core plastic optical fiber strands, as multi-core plastic optical fibers which can withstand the environment in the high-temperature region of the order of 80 to 110° C., which are made up of: a core resin of poly(methyl methacrylate)-based; and a cladding resin of vinylidene fluoride/tetrafluoroethylene/hexafluoropropene tertiary polymer having a Shore D hardness in the range of 30 to 55 (see Patent Document 2).
There have also been proposed plastic optical fiber cables, as plastic optical fiber cables having excellent heat resistance, flame resistance, oil resistance and chemical resistance, which are produced by coating the outside of plastic optical fiber strands consisting of a core and a cladding with fluorine-containing polyolefin resin and further with nylon 12 resin (see Patent Document 3).    [Patent Document 1] Japanese Patent Laid-Open No. 2003-244295    [Patent Document 2] Japanese Patent Laid-Open No. 11-95048    [Patent Document 3] Japanese Patent Laid-Open No. 7-77642